1. Field of the Invention
The present invention relates to a resist lower layer film material of a multilayer-resist film used in lithography, and especially to a resist lower layer film material of a multilayer-resist film suitable for exposure with far ultraviolet rays, ArF excimer laser light (193 nm), F2 laser light (157 nm), Kr2 laser light (146 nm), Ar2 laser light (126 nm), or the like. Furthermore, the present invention also relates to a method for forming a pattern on a substrate by lithography using it.
2. Description of the Related Art
With a tendency of high integration and high-speed of LSI, a finer pattern rule is needed in recent years, and in lithography using optical exposure which is used as a general technique at a present, an essential resolution derived from a wavelength of a light source has almost reach the limit.
There has been used widely optical exposure using g line (436 nm) or i line (365 nm) of a mercury-vapor lamp as a light source for lithography when a resist pattern is formed. It has been considered that a method of using an exposure light with a shorter wavelength is effective as a means for a further finer pattern. For this reason, for example, KrF excimer laser (248 nm) with a short wavelength has been used as an exposure light source instead of i line (365 nm), for mass-production process of a 64 M bit DRAM processing method. However, a light source with far shorter wavelength is needed for manufacture of DRAM with a degree-of-integration of 1 G or more which needs a still finer processing technique (for example, a processing dimension is 0.13 μm or less), and lithography using ArF excimer laser (193 nm) has been especially examined.
On the other hand, it has been known conventionally that a multilayer-resist process such as a two-layer resist process is excellent in order to form a pattern with a high aspect ratio on a board which has a difference in level. Especially, it is supposed that it is preferable to use a high-molecular silicone compound having a hydrophilic group, such as a hydroxy group, a carboxyl group and the like, as a resist upper layer film, in order to develop a two-layer resist film with a general alkali developer in a two-layer resist process.
As such a high-molecular silicone compound, there have been proposed for KrF excimer lasers a silicone chemically amplified positive-resist material wherein polyhydroxy benzyl silsesquioxane, which is a stable alkali soluble silicone polymer, in which some phenolic hydroxyl groups are protected by a t-Boc group is used as a base resin, and it is combined with an acid generating agent (for example, see Japanese Patent Application Laid-open (KOKAI) No. 6-118651 and SPIE vol. 1925 (1993) p 377). Moreover, there have been proposed for ArF excimer lasers a positive resist wherein silsesquioxane of which a cyclohexyl carboxylic acid is substituted with an acid unstable group is used as a base (for example, see Japanese Patent Application Laid-open (KOKAI) No. 10-324748, Japanese Patent Application Laid-open (KOKAI) No. 11-302382, and SPIE vol. 3333 (1998) p 62). Furthermore, there has been proposed for F2 laser a positive resist wherein silsesquioxane having a hexafluoro isopropanol as a soluble group is used as a base (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2002-55456). These high-molecular silicone compounds contain poly silsesquioxane containing a ladder frame by the condensation polymerization of a trialkoxy silane or a tri halogenated silane in a backbone chain.
As a base polymer for resist wherein silicon is suspended from a side chain, (meta)acrylic ester polymer containing silicon has been proposed (for example, see Japanese Patent Application Laid-open (KOKAI) No. 9-110938 and J. Photopolymer Sci. and Technol. Vol. 9 No. 3(1996) p 435–446).
Examples of a resist lower layer film used for a multilayer-resist process, such as a two-layer resist process may include a hydrocarbon compound which can be etched with oxygen gas and the like. It is desirable to have a high etching resistance, since it is used as a mask in the case of etching a substrate under it further. When etching of the resist lower layer film using as a mask the resist upper layer film is conducted according to oxygen gas etching, it is preferable that the resist lower layer film consists of only hydrocarbons which do not contain a silicon atom. Moreover, in order to improve a line width controllability of the resist upper layer film containing a silicon atom and to reduce irregularity on a pattern side wall due to a stationary wave and collapse of a pattern, it is preferable that the resist lower layer film also has a function as an antireflection film. Specifically, it is desirable that a reflectivity from the lower layer film to the resist upper layer film can be kept at 1% or less.
The anti-reflection film as a ground for a monolayer resist process can reduce reflectivity from the substrate to 1% or less by being formed of the material having an optimal refractive index (n value) and an optimal extinction coefficient (k value) with a suitable thickness, even when the substrate under it is a highly reflective board such as polysilicon, aluminum or the like, and thereby quite high anti-reflection effect can be achieved.
FIG. 1 is a graph which shows a relation between a thickness of an anti-reflection film as a ground for a monolayer resist process and a reflectivity thereof at a wavelength of 193 nm. FIG. 1 shows that, for example, in the case that a refractive index of a photoresist film is 1.7 at a wavelength of 193 nm, if the refractive index of the anti-reflection film under it (a real number part of a refractive index) n is 1.5, an extinction coefficient (a imaginary number part of a refractive index) k is 0.5, and a thickness is 42 nm, a reflectivity will become 0.5% or less.
However, in the case that there is a level difference in a ground substrate, a thickness of an anti-reflection film is sharply changed at the level difference. Although the reflection preventive effect in the first base with a thickness of 40–45 nm where the interference effect is strong is high, a reflectivity is sharply changed due to variation of a thickness, since the reflection preventive effect of the anti-reflection film uses not only an absorption of light but the interference effect due to setting an optimal thickness, as shown in FIG. 1.
Then, there has been proposed the material wherein a thickness variation on a level difference is suppressed by increasing a molecular weight of a base resin used for the anti-reflection film material, and thereby a conformal property is improved (for example, see Japanese Patent Application Laid-open (KOKAI) No. 10-69072) However, if the molecular weight of the base resin becomes high in this case, there may be caused a problem that a pinhole is easily generated after spin coating, a problem that it becomes impossible to be filtered, and further a problem that viscosity change will be caused with time and a thickness will be varied, and a problem that a crystal is deposited at a tip of a nozzle. Moreover, a conformal property can be achieved only where a level difference is comparatively low.
Then, there can be considered a method of adopting a thickness more than that of the 3rd base line (170 nm or more) where variation of a reflectivity due to variation of a thickness is comparatively small from FIG. 1. In this case, if k value is between 0.2–0.3, and a thickness is 170 nm or more, the variation of a reflectivity due to variation of a thickness will be small, and a reflectivity can be suppressed to 1.5% or less. However, taking an etching load of the resist film on it into consideration, there is a limitation in making an antireflection film thick, and a thickness of the 2nd base of only 100 nm or less at most is possible.
Moreover, in the case that the ground of the anti-reflection film is a transparent film, such as an oxide film, a nitride film or the like, and there is a level difference under the transparent film, even though the surface of the transparent film was made flat by CMP (Chemical Mechanical Polishing) or the like, a thickness of the transparent film is varied. In this case, although it is possible to make a thickness of the anti-reflection film on it constant, a thickness of the film wherein a reflectivity is minimum will shift by the thickness of the transparent film, when the thickness of the transparent film under the anti-reflection film is varied. Even if a thickness of the anti-reflection film is set so that a reflectivity in the case that the ground is a reflective film may be a minimum value, a reflectivity gets high due to variation of a thickness of a transparent film in some cases.
The material of the anti-reflection film as described above can be roughly classified into an inorganic material and an organic material.
Example of the inorganic material may be a SiON film. Since it is formed by CVD (Chemical Vapor Deposition) with a mixed gas of silane and ammonia and an etching selectivity to a photoresist film is large, it has an advantage that an etching load to the resist film is small. However, since it is hardly exfoliated, application thereof is limited. Moreover, since it contains a nitrogen atom and is a basic substrate, there is also a disadvantage that the footing is likely caused in the case of a positive resist, and an undercut profile is easily caused in the case of a negative resist.
The organic material is advantageous, since it can be formed by a spin coating, it does not need special equipments, as in CVD, sputtering or the like, it can be stripped together with the resist film, footing profile or the like is not generated, the shape is simple, and it has an excellent adhesion property with the resist film. Accordingly, a lot of anti-reflection film materials based on organic materials were proposed. For example, there were proposed those consisting of a condensation product of a diphenylamine derivative and a formaldehyde denaturated melamine resin, an alkali soluble resin and a light absorber (for example, see Japanese Patent Publication No. 7-69611), a reaction product of a maleic anhydride copolymer and diamine type light absorber (for example, see U.S. Pat. No. 5,294,680 specification), those containing a resin binder and a methylol melamic heat crosslinking agent (for example, see Japanese Patent Application Laid-open (KOKAI) No. 6-118631), an acrylate resin base type which has a carboxylic acid group, an epoxy group, and a light-absorption group in the same molecule (for example, see Japanese Patent Application Laid-open (KOKAI) No. 6-118656), those consisting of methylol melamine and a benzophenone light absorber (for example, see Japanese Patent Application Laid-open (KOKAI) No. 8-87115), those wherein a low molecule light absorber is added to a polyvinyl alcohol resin (for example, see Japanese Patent Application Laid-open (KOKAI) No. 8-179509), and the like. All of the anti-reflection film materials using these organic materials as a base are produced by a method of adding a light absorber into a binder polymer, or introducing a light-absorption group into a polymer as a substituent. However, since many of the light absorbers have an aromatic group or a double bond, dry etch resistance is raised by addition of the light absorber, and thus there is a disadvantages that a dry etching selectivity to the resist film is not so high. Since a pattern tends to be finer, a photoresist film tends to be thin, and furthermore, an acrylic or an alicyclic polymer will be used as a photoresist film material in ArF lithography of the next generation, etching resistance of the photoresist film tends to be lowered. Furthermore, there is also a problem that a thickness of the anti-reflection film needs to be increased, as explained above. For these reasons, etching is becoming a serious problem, and thus the anti-reflection film with high etching selectivity to the resist film, namely the anti-reflection film wherein an etch rate is high has been desired.
On the other hand, the function demanded as an antireflection film for the resist lower layer film for a multilayer-resist process such as a two-layer resist process is different from those for the antireflection film of a monolayer resist process. Since the resist lower layer film for the two-layer resist process serves as a mask when etching a substrate, it needs to have a high etching resistance under the condition of etching of the substrate. Thus, a high etch rate is required for an antireflection film in a monolayer resist process in order to make an etching load of the monolayer resist film light, whereas the contrary characteristics are required for a resist lower layer film in a multilayer resist process. Moreover, in order to secure a sufficient etching resistance at the time of etching the substrate, it is necessary to make a thickness of the resist lower layer film as thick as a monolayer resist film, or more, namely as 300 nm or more. However, with a film thickness of 300 nm or more, change in a reflectivity due to change in a film thickness is almost converged, and the reflection preventing effect by control of phase difference cannot be expected.
The results of a calculated reflectivity of a substrate when a thickness of a resist lower layer film was changed in the range of 0–500 nm are shown in FIGS. 2 and 3. It is assumed that an exposure wavelength is 193 nm, n value of the resist upper layer film is 1.74, and k value is 0.02.
FIG. 2 shows a reflectivity of the substrate when k value of the resist lower layer film is fixed to 0.3, a vertical axis represents n value, a horizontal axis represents a thickness, the n value is varied in the range of 1.0–2.0, and a thickness is varied in the range of 0–500 nm. As shown in FIG. 2, assuming the resist lower layer film for a two-layer resist process with a thickness of 300 nm or more, there exists an optimal value where a reflectivity can be about 1% or less, when an index of refraction (n value) is as high as, or slightly higher than the resist upper layer film, namely is 1.6–1.9.
FIG. 3 shows a reflectivity when n value of the resist lower layer film is fixed to 1.5, a vertical axis represents k value, a horizontal axis represents a thickness, the k value is varied in 0–0.8, and a thickness is varied in 0–500 nm. As shown in FIG. 3, assuming the resist lower layer film for a two-layer resist process with a thickness of 300 nm or more, a reflectivity can be about 1% or less when k value is 0.24–0.15. On the other hand, the optimal k values of the antireflection film for a monolayer resist process used as the thin film as about 40 nm are 0.4–0.5, and differ from the optimal k value of the resist lower layer film for a two-layer resist process of which a thickness is 300 nm or more. As described above, it is shown that the lower k value (i.e., higher transparency) is necessary for the resist lower layer film for a two-layer resist process.
Then, a copolymer of a polyhydroxy styrene and an acrylic has been examined as a resist lower layer film material for a wavelength of 193 nm (for example, see SPIE Vol. 4345 p 50 (2001)). Polyhydroxy styrene has a very strong absorption at a wavelength of 193 nm, and the k value of itself is as high as about 0.6. Then, the k value is adjusted around 0.25 by carrying out copolymerization with the acrylic of which k value is almost 0.
However, etching resistance of acrylic is low at etching of the substrate, compared with etching resistance of polyhydroxy styrene, and the acrylic need to be copolymerized at a significant rate in order to lower the k value. As a result, the etching resistance at the time of etching of the substrate is significantly lowered. The etching resistance appears not only in an etch rate but in generation of surface roughness after etching. The increase of surface roughness after etching becomes serious due to copolymerization of the acrylic.
Then, it has been proposed to use a naphthalene ring which is one of those wherein a transparency at a wavelength of 193 nm is higher than a benzene ring and etching resistance is high. For example, there has been proposed a resist lower layer film which has a naphthalene ring or an anthracene ring (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2002-14474). However, the k value of a naphthol copolycondensation novolak resin and a polyvinyl naphthalene resin is between 0.3 and 0.4, and does not reach a desired transparency of 0.1 to 3, and thus it is necessary to raise a transparency further in order to achieve a desired reflection preventing effect. Moreover, the n value at a wavelength of 193 nm of a naphthol copolycondensation novolak resin and polyvinyl naphthalene resin is low, and it is 1.4 in the case of a naphthol copolycondensation novolak resin, and is 1.2 in the case of a polyvinyl naphthalene resin as measured by the inventors of the present invention, which do not reach the desired range of value. Furthermore, although the acenaphthylene polymer is proposed (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2001-40293 and Japanese Patent Application Laid-open (KOKAI) No. 2002-214777), the n value at a wavelength of 193 nm is lower than at a wavelength of 248 nm, k value is high, and neither of the desired values is achieved.
As described above, there is a need for the resist lower layer film which has the high n value and the low k value, is transparent, and has a high etching resistance.